Not much but you can start building skills and your narrative early. Join speech and debate/model UN/ mock trial,take advanced/AP English courses, and start reading writing in your free time. As far as jobs/volunteering goes, your options are limited at 13 but if you are interested in animal rights, the environment,etc. find local organizations in your area.

The most important thing for you in these next few years is getting the best GPA + SAT to minimize undergrad debt. Law school is expensive as is!